The present invention relates to a method and apparatus of removing moisture from air passing through an air cooling system.
The population in the U.S. is shifting toward the Southern part of the country. National Association of Home Builders figures indicates that out of 1.7 million housing starts in 1983, approximately 67.8% occurred in the South. Of the housing starts in the South, it is estimated that about 62% occurred in the Southeastern U.S., which generally experiences long, hot, humid summers. As more and more homes are built in the South, cooling energy use increases. For example, the average Florida home uses 30% of needed energy for cooling.
Alternatives have been introduced throughout the United States to reduce the use of cooling energy. Thermostats have been raised in summer and considerable work has been completed in the area of passive cooling to provide reduced energy loads and maintain comfortable conditions; radiant barriers, ceiling fans, vent-skin walls, vent-skin roofs and enhanced ventilation techniques are among these. These techniques cannot, however, remove the moisture load on the building environment. Under typical construction techniques and infiltration, the moisture in the humid Southeastern USA may comprise 30%-40% of the overall AC load. However, as conserving building techniques are incorporated to reduce the sensible heat load, the moisture or latent load may reach over 60% of the cooling energy required to maintain comfort conditions.
Dehumidifiers that are presently commercially available are relatively inefficient. The cooling energy of a particular evaporator coil design is part sensible (heat removal), and part latent (moisture removal). The ratio of sensible heat removed to the total cooling energy is called the sensible heat ratio (SHR) and is fixed by the AC design for any operation inputs. A typical AC unit cannot dehumidify more than its coil design ratio. So to remove more moisture by traditional air conditioning, one has to lower the thermostat causing excessive cooling. This is not only counter productive to the passive cooling techniques that may be employed, but also may result in uncomfortable, cold-clammy environments and cause excessive energy use.
The improvement of the energy efficiency (SEER) of air conditioner equipment by manufacturers has been undertaken to reduce energy consumption. This has been accomplished by enlarging the heat exchange coils utilized by the improved AC equipment. When such equipment is installed properly i.e., installations are down sized from normal sizing such that short-cycling does not occur, energy efficiency is improved. The larger coils, however, have the negative side effect of increasing the sensible to latent operating ratio of the equipment. They thus remove less moisture and exacerbate the dehumidification problem.
To remove moisture from air, it must first be cooled to its dew point. A typical dehumidifier utilizes the condenser heat available in its condenser coil to reheat the air that has been cooled to its dew point temperature for dehumidification purposes. This reheat delivers the air at the desired temperature to the space. While this process is workable, it does not increase the efficiency of the apparatus. In some systems, primarily commercial, the reheat is provided via electrically driven resistance coils. This approach is workable but is energy inefficient.
It is also well known that heat exchangers can be utilized in connection with ventilating and air-conditioning devices. These devices usually include a means for bringing in fresh outside air and transferring heat from the incoming outside air to the exhausted inside air, thus reducing the heat load of the fresh air and providing reheat. This process, however, brings in moisture associated with the outside air.
Recent advances in the application of heat exchangers to AC equipment for dehumidification purposes have been made by Earl Doderer of Trinity University of San Antonio, Tex., and Mukesh Khattar of the Florida Solar Energy Center in Cape Canaveral, Fla.
The Doderer system as set forth in U.S. Pat. No. 4,428,205, utilizes a cross flow heat exchanger to provide heat transfer from the hotter room air being drawn toward the AC cooling coil. The heat passes to the chilled air leaving the coil going to the room. Thus, reheat is provided from air entering the equipment at no additional energy expense. Thus, an energy efficiency is achieved in the dehumidification process. To obtain the proper heat transfer very large heat exchange surfaces are necessary.
Mukesh Khattar has analyzed the application of heat pipe technology to this heat transfer application. This application was originally proposed by Kahn Dinh of Gainsville, Fla. Rather than large cross flow plenem-type exchangers, heat pipe coils similar to those commonly in use by the HVAC industry are employed. Though both these methods improve dehumidification efficiency, some efficiency gain is lost due to the increased fan power requirements. The energy gain is also limited to sensible heat transfer which provides for colder cooling coil operating temperatures reducing the operational COP of the equipment.
The application of desiccants to utilize heat transfer through heat of sorption and desorption as well as direct moisture transfer show promise for major dehumidification efficiency improvements over standard AC equipment with a minimal increase in fan power and improved COP.
Prior art may be seen in the Newton U.S. Pat. No. 2,811,223 which shows a pair of heat exchangers with a dryer in front of each and a water sprayer tacked onto the second heat exchanger. The Northrup, Jr. U.S. Pat. No. 4,180,985 shows a desiccant dryer used in connection with a pair of cooling coils for removing moisture from the system. The Griffiths U.S. Pat. No. 4,164,125 shows a solar energy assisted air conditioning system which disperses a shower of moisture into the cooling coil and then collects the condensed moisture for later use. The Turner U.S. Pat. No. 4,171,620 provides for dehumidification by contact of the air with a hygroscopic solution. A pair of Rush U.S. Pat. Nos. 4,180,126 and 4,081,024 each show air conditioning systems using drying wheels.